A conventional tape transport device, such as the tape transport device used in a video tape recorder/player 10, is schematically depicted in FIG. 1a. Tape 11, which is supplied from a supply reel 12, is fed between a capstan 13 and pinch roller 14 assembly to a take-up reel 15. In FIG. 1b, a cross-sectional segment of the tape 11 is shown having a thickness t. The tape itself has a total lenght L and L&gt;&gt;t. For example, t=20 micrometers (.mu.m) and L=248 meters (m) for a T-120 VHS cassette tape. The transport of tape 11 is controlled by a capstan drive motor 16 connected to the capstan-pinch roller assembly 13-14. In the operation of the recorder/player, tape 11 is advanced from supply reel 12 to take-up reel 15 at one of at least two alternative speeds by capstan motor 16. Illustratively in video tape recording commonly used speeds are designated as SP mode (Standard Play speed) and EP mode (Extended Play speed), where the speed ratio of SP mode to EP mode is typically 3:1.
When a standard video tape in VHS format is rated for 120 minutes (designated T-120), the recording time is 120 minutes at Standard Play (SP) speed, or 360 minutes at Extended Play (EP) speed. The major disadvantage of recording at EP speed is that the signal-to-noise ratio (S/N) is inferior to that at SP speed. Therefore, for maximum recording fidelity, the preferred mode is SP speed. In the event that a source program exceeds 120 minutes, but does not exceed 360 minutes, a single T-120 tape may be used, through the judicious selection of SP and EP speeds. Therefore, to maximize fidelity and to complete the recording within a single T-120 tape, the recording process is started in the SP mode, and is then switched to the EP mode when the remaining source program time just equals the remaining tape time at EP speed. Moreover, the speed change is implemented automatically, to eliminate the otherwise difficult and inconvenient task of manual control by an operator.
To determine the remaining source program time, a count-down timer is typically used to progressively decrease the initial source program time by the amount of elapsed recording time. To determine the remaining tape time, the rotational period of the supply reel is measured by a detector 17 (reel rotation sensor 19 and counter 20) and processor 18 combination, then the remaining tape length is determined, and finally, the remaining tape time is determined, based on the known tape speed. These determinations may be made according to equations established by the prior art, as follows: EQU r.sub.s =(V.sub.t *T.sub.sy)/2.pi. Equation (1)
where r.sub.s is the outer radius of the supply reel tape (FIG. 1a), V.sub.t is the tape speed, and T.sub.sy is the rotational period of the supply reel. Then, EQU L.sub.s=.pi. (r.sub.s .sup.2 -r.sup.2)/t Equation (b 2)
where L.sub.s is the remaining tape length on the supply reel, r is the hub radius of the supply reel, and t is the tape thickness (see FIG. 1a,b). Finally, EQU T.sub.s =L.sub.s /V.sub.t Equation 3)
where T.sub.s is the remaining tape time on the supply reel at the speed V.sub.t.
In U.S. Pat. No. 5,032,937, issued on Jul. 16, 1991 (Suzuki et al.), an automatic mode change technique is disclosed, which is based on the continuous calculation of the remaining tape time, at SP speed, on the supply reel. When the remaining source program time is equal to 3 (or n) times this continuously calculated remaining tape time, the tape transport is switched to EP speed. This technique, however, requires the inclusion of complex hardware and software to achieve the continuing "on-the-fly" calculation of remaining tape time throughout the first (SP) speed operational mode.
To overcome this prior art disadvantage, it is an object of the present invention to achieve the benefits of recording fidelity and operator convenience with an automatic mode (speed) change capability, but without the additional hardware complexity required for the prior art continuous calculation of remaining tape time on the supply reel.